GENERATORS: Add a test for zero-beginning signal generation

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
TESTS: Expose GetFirst function

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
GENERATORS: Loosened perf tests

Since these must always pass - they do not check anything - they are
loosened here to take into account some rare imprecision cases where
the output may be temporary outside the normalized range.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>